1. Field of the Invention
The present invention relates to a hydraulic clutch actuator for a torque-coupling device, such as a speed sensitive limited slip differential for use with a vehicle drive train, and more particularly to a piston member of the hydraulic clutch actuator.
2. Description of the Prior Art
Conventionally, differentials well known in the prior art, are arranged in a power transmission system of a motor vehicle to allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, these types of differentials known in the art as an open differentials, i.e. a differential without clutches or springs, are unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel; for instance, when one wheel of a vehicle is located on a patch of ice or mud and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a xe2x80x9cspin outxe2x80x9d of that wheel. Since the maximum amount of torque, which can be developed on the wheel with traction, is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. Thus, the necessity for a differential, which limits the differential rotation between the output shafts to provide traction on slippery surfaces, is well known.
A number of devices have been developed to limit wheel slippage under such conditions. Conventionally, they use a frictional clutch between the side gears and the differential casing. The frictional clutch may be selectively actuated by various hydraulic actuator assemblies, which are constructed of elements disposed inside the differential casing. Such differential assemblies are typically called limited slip differentials.
The hydraulic actuator assemblies of the limited slip differentials, as well as many other types of hydraulically actuated friction coupling have utilized internal gear sets, which are often called gerotors. Such devices can be used as pumps where rotational work is converted to hydraulic work. In the internal gear pumps, an inner gear having outwardly directed teeth cooperates with an external gear having inwardly directed teeth so that fluid chambers therebetween increase and decrease in volume as the inner and outer gears rotate in a housing. By connecting the inlet and outlet of the device to the proper location along the sides of the gear set, the variable displacement chambers receive and discharge hydraulic fluid so that the device can function as a pump or motor. A shaft or other mechanical device can be connected to either the inner or outer gear depending upon the type of device. The hydraulic actuator assemblies further include a hydraulic piston member for frictionally loading the friction clutch. The typical piston member of the prior art is housed in a pump housing insert having a finely machined sealing surface for receiving the hydraulic piston. Current hydraulic pistons are themselves expensive in manufacturing for they are provided with seals having delicate seal lips. The piston has to be assembled carefully in the housing insert in order to not damage the seal lips that makes such a device rather complex, expensive, laborious in assembly. Furthermore, the existing hydraulic actuators typically require a separate spring element providing an initial preload of the hydraulic piston.
The present invention provides an improved hydraulically actuated friction coupling.
The hydraulically actuated friction coupling in accordance with the present invention includes comprises a hollow case, at least one output shaft drivingly connected to the case, a friction clutch pack for selectively engaging and disengaging the case and the output shaft, and a hydraulic clutch actuator for selectively frictionally loading the clutch pack. The hydraulic actuator includes a hydraulic pump located within the case for generating a hydraulic pressure to frictionally load the clutch pack and a novel piston assembly disposed within the case between the pump and the clutch pack. The piston assembly includes an annular pressure plate adjacent to the pump, an annular thrust plate axially spaced from the pressure plate and adjacent to the clutch pack, and a pair of concentric radially spaced resilient annular seal members bonded to the pressure and thrust plates so as to define an enclosed annular cavity within the piston assembly. The pressure plate has one or more inlet openings theretrough for providing fluid communication between the annular cavity and a space within the differential case between the hydraulic pump and the piston assembly. The inlet opening is disposed adjacent to a discharge port of the pump. Preferably, the piston assembly is non-rotatably coupled to the differential case.
Preferably, the hydraulic pump is a gerotor pump disposed within the case and generating a hydraulic pressure in response to relative rotation between the case and the output shaft.
The friction clutch pack includes a number of alternating inner friction plates non-rotatably coupled to the output shaft, and outer friction plates non-rotatably coupled to the case. The friction clutch pack is axially loadable by the piston assembly to cause a frictional restriction of rotation of the output shaft relative to the case.
In accordance with the preferred embodiment of the present invention the hydraulically actuated friction coupling of the present invention is incorporated into a limited slip differential assembly comprising a differential case, a differential gear mechanism, a friction clutch pack and a hydraulic clutch actuator, as described hereinabove.
In operation, the pressurized hydraulic fluid, preferably an axle lubricant, generated in response to relative rotation between the differential case and the output shaft, enters the annular cavity through the inlet openings in the pressure plate. Under the pressure of the hydraulic fluid, the piston assembly expands axially by moving the thrust plate away from the pressure plate toward the clutch pack in the axial direction and presses the friction plates of the clutch pack so as to gradually bias the differential assembly. When the clutch pack is clutched, the relative rotation between the output axle shaft and the differential case is limited so that the differential rotation of the differential mechanism is limited.
Therefore, the hydraulically actuated friction coupling in accordance with the present invention uses less space within the case, eliminates the need for a separate piston housing, seal lips and a separate spring element providing an initial preload of the hydraulic piston of the prior art, and, thus, is simple, compact, and less expensive and laborious in manufacturing than existing hydraulically actuated friction couplings.